Schistosomiasis per se is recognized as one of the most important of the parasitic diseases, but there are great voids in the information on the basic biology of some types of parasites responsible for this disease, especially with infections due to S. haematobium, S. intercalatum, and allied species of this complex. Although hamsters and mice serve as the principal sources for research materials, little is known of the comparative biology of these schistosomes in their definitive hosts. In the coming period, studies will be made with S. haematobium, S. intercalatum, and other species 8, 16, 24, and 36 weeks post infection to determine basic host-parasite relationships such as parasite production, egg deposits in vital organs, and histopathology dependent upon the latter, and to determine the feasibility of use of certain hosts as models for continuing multidisciplinary investigations. Since some members of the terminal spine egg schistosome complex have a proven potential as carcinogens for bladder carcinoma, observations of S. intercalatum and other schistosomes in hamsters will be coupled with investigations on the same parasite(s) in nonhuman primates. The first in a series of steps will be observations on the parasitology and histopathology of S. intercalatum infection in several primates which are already patent and are now in the intermediate stages of infection.